Analysis and design of a three-phase constant voltage transformer based on ferroresonance

Applications requiring a constant terminal voltage may benefit from using a distribution transformer based on ferroresonance. This transformer guarantees an output voltage complying with all standards for large fluctuations in input voltage. This offers significant advantages over on-load tap changer control for certain applications, and can be an alternative for power electronic based systems. In this paper the operation of a three-phase constant-voltage-transformer (CVT) is analysed, its behaviour is modelled, and this model is used in simulations. A low power prototype has been built to validate the design equations. Measurements are compared to simulation results to test the accuracy of the numerical model. An excellent agreement is found between experiments and calculated results if component losses are taken into account properly. The functionality of the prototype is evaluated with respect to efficiency, harmonic distortion, power factor and unbalanced operation. The influence of different loading conditions has also been investigated. The prototype is found to work properly up to a maximum power demand, both for resistive as for partly capacitive or inductive loads.